Journal of Electroanalytical Chemistry 661 (2011) 322–328

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Journal of Electroanalytical Chemistry

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Amperometric determination of sodium hypochlorite at poly MnTAPP-nano Aufilm modified electrode

Soundappan Thiagarajan, Zhi-Yuan Wu, Shen-Ming Chen ⇑Electro Analysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1, Section 3,Chung-Hsiao East Road, Taipei 106, Taiwan, ROC

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 July 2010Received in revised form 8 July 2011Accepted 11 August 2011Available online 1 September 2011

Keywords:Poly MnTAPPNano AuSodium hypochloriteCyclic voltammetryAmperometry

1572-6657/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jelechem.2011.08.009

⇑ Corresponding author. Tel.: +886 2 27017147; faxE-mail address: smchen78@ms15.hinet.net (S.-M.

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.cPoly manganese tetra (o-amino phenyl) porphyrin (MnTAPP)-nano Au film was electrodeposited onglassy carbon electrode (GCE) and indium tin oxide coated glass electrode (ITO) using cyclic voltammetry(CV). Electrodeposited poly MnTAPP-nano Au modified electrodes have been characterized using scan-ning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Poly MnTAPP-nano Aufilm modified GCE found as electrochemically active and stable. Poly MnTAPP-nano Au modified GCEemployed for the amperometric detection of sodium hypochlorite (NaOCl) in the linear range of24 � 10�6–1.07 � 10�2 M. The proposed film holds the capacity to overcome the interference substancesand showed the reduction current response for the NaOCl additions. In addition, the poly MnTAPP-nanoAu film successfully exhibits the reduction current of NaOCl concentrations in the commercially availablebleaching liquids, respectively.

� 2011 Elsevier B.V. All rights reserved.

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www.sp1. Introduction

Sodium hypochlorite (NaOCl) solution is a strong oxidizer, com-monly known as bleach, frequently used in household purpose asdisinfectant or bleaching agent. Generally, household bleachingagents used for laundering cloths contains 3–6% solution of sodiumhypochlorite. Here the NaOCl strength varies from one product toanother one. NaOCl is widely used in water works for the chlorina-tion of water and as disinfectant for the waste water treatment.Daily sanitizing sprays contain NaOCl as an active ingredient. Atthe same time, the oxidation reactions of NaOCl were corrosive.Contact with NaOCl solutions may directly affect the skin, irritationin eyes, which results in the damage of human organs. Therefore,analytical detection and determination of NaOCl is important onein our routine life [1,2].

Various analytical methods have been utilized for the determi-nation of NaOCl. Some of the examples were colorimetric [3], iodi-metric [4], chemiluminiscent [5], potentiometric [6], and greenmethods [7]. All these methods are found as suitable for the directdetection of NaOCl in the chemical laboratories. However, detailedstudies for the detection and determination of NaOCl using electro-analytical methods will help to fabricate the commercially avail-able proto type electrochemical sensor which will be reliable andeasy to use and cost effective one.

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: +886 2 27025238.Chen).

Manganese tetra (o-amino phenyl) porphyrin (MnTAPP) con-tains four branched monomers (Scheme 1) and its polymerizationresults in the formation of dentrimer [8]. ElectropolymerizedMnTAPP and MnTAPP adsorbed flavin hybrid films were reportedfor the various types of electrocatalytic reactions [8,9]. Next, theelectrochemical depositions of gold nanoparticles (nano Au) werefound as interesting phenomena in the new era. Gold nanoparticlesplay important role in the fabrication of nanostructured assem-blies. Nanostructured Au shows higher electrocatalytic activitythan the bulk Au [10,11]. Particularly, the desired shape and sizeascertain obvious changes in their catalytic activities [12,13].Therefore, utilization of gold nanoparticles was found as usefulfor the development of electrochemical sensors and biosensors.For instance, flower-shaped Au nanoparticles exhibit obvious elec-trocatalytic properties for the oxidation of methanol, oxygenreduction reactions and the detection of H2O2 [14,15]. In particular,fabrication of nanomaterial anchored metalloporphyrins wasfound as new film materials in the field of electroanalytical chem-istry. Hence, in this report we have attempted to fabricate the elec-tropolymerized porphyrins–nano Au film modified electrodes.Cyclic voltammetry (CV) was utilized for the electrochemical fabri-cation of poly MnTAPP-nano Au film. Poly MnTAPP-nano Au filmshave been fabricated on glassy carbon and indium tin oxide elec-trodes (ITO). Electropolymerized MnTAPP, nano Au and polyMnTAPP-nano Au films were characterized using SEM and AFManalysis. Poly MnTAPP-nano Au film modified GCE was employedfor the amperometric detection of NaOCl. It showed good responsefor the detection and determination of NaOCl in lower and higher

mScheme 1. Structure of MnTAPP.

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concentration ranges. Not only limited to lab samples, polyMnTAPP-nano Au film detects NaOCl in commercially availabledetergents within the linear range limits, respectively.

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2. Materials and methods

2.1. Reagents

KAuCl4�3H2O was obtained from Strem chemicals (USA). TheMn (o-TAPP) was purchased from Porphyrin Products (Logan, Utah,USA). NaOCl (5% chlorine) was purchased from Wako ChemicalsLtd. (Japan). Other chemicals were of analytical grade. Double

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Fig. 1. (A) Electrochemical deposition of poly MnTAPP-nano Au film on GCE from pH 1.015 cycles. (Potential scan range: 1–0 V). Insets (B and C) represent the electrochemical derespectively.

distilled deionized water was used throughout the experiments.All the experimental results were obtained at room temperature.Commercially available bleaching liquid (containing NaOCl) hasbeen obtained from local departmental store. NaOH buffer solution(pH 10.4) was utilized for the electrocatalytic reactions. Pure nitro-gen was passed through all the experimental solutions.

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2.2. Apparatus

Electrochemical measurements like cyclic voltammetry (CV)and amperometric experiments were performed using CHI 627aand CHI 750 Potentiostats (CH Instruments, Austin, TX). A conven-tional three-electrode system was used throughout the experi-ments. Glassy carbon electrodes (/ = 0.3 cm in diameter) (BASi,USA) were in the form of disks sealed in a Teflon jacket havingan exposed geometric surface area of 0.07 cm2. The working elec-trode was bare or poly MnTAPP-nano film Au modified GCE. Theauxiliary electrode was platinum wire and Ag/AgCl electrode wasused as a reference. All the potentials mentioned in this paper werereferred to this reference electrode. The morphological character-izations of the film were studied by using SEM (Hitachi S-3000H)and AFM (Being Nano-Instruments CSPM-4000). UV–vis spectrawere obtained using Hitachi U-3300 spectrophotometer (Japan).Here, indium tin oxide (ITO) thin film coated glass electrodes wereused for AFM and UV analysis.

.co2.3. Fabrication of poly MnTAPP-nano Au film modified GCE

The bare GCE was polished with the help of a BAS polishing kitusing aqueous slurries of alumina powder (0.05 lm), rinsed andultrasonicated in double distilled deionized water. 1 mM KAuCl4

and 0.1 mM MnTAPP were prepared in pH 1.0 H2SO4 solution. Laterthe stock solution was prepared by mixing 3 ml of 1 mM KAuCl4

with 1 ml of 0.1 mM MnTAPP. Poly MnTAPP-nano Au film has beenelectrochemically fabricated on GCE by applying repetitive poten-tial scan between 1 and 0 V at the scan rate of 0.1 V/s for 15 cycles

H2SO4 containing 0.1 mM MnTAPP and 1 mM KAuCl4 at the scan rate of 0.1 V/s forposition of only poly MnTAPP film and electrochemical deposition of Au at the GCE,

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[15]. Poly MnTAPP-nano Au film modified GCE washed with deion-ized water and employed for the detailed electrochemical analysis.

3. Results and discussion

3.1. Characterization of poly MnTAPP-nano Au film modified GCE

Fig. 1A shows CVs of the electrochemical fabrication of polyMnTAPP-nano Au in aqueous H2SO4 solution (pH 1) containing0.1 mM MnTAPP and 1 mM KAuCl4. At the initial scan, the peakaround 0.5 V depicts the reduction of Au3+ and another one reduc-tion peak shows the electrodeposition of poly MnTAPP. For thecontinuous cycles, this reduction peak shifts to positive potential(at 0.66 V). Also, this reduction peak overlay with the third reduc-tion peak of poly MnTAPP. For the continuous cycles, the growth ofthe CV showed two redox couples with the formal potentials ofE00 = 0.69 and 0.48 (vs. Ag/AgCl) indicates the Mn(II)/Mn(III) redoxcouple and the porphyrin ring of MnTAPP [8,9].

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Fig. 2. The typical SEM image obtained for the electrodeposited (a) poly MnTAPP, (b) el(view angle: 60�).

Here the third redox couple (E00 = 0.69 V) was not obvious be-cause the co-deposition of nano Au [15]. The sharp oxidation peakaround 1 V resembles the oxidation process of both Au and polyMnTAPP. This result has been verified by the individual electrode-position of poly MnTAPP (Fig. 1 Inset (B)) and Au (Fig. 1 Inset (C))modified GCEs, respectively. Comparing the Fig. 1A with the Fig. 1Band C, it can be clearly seen that the electrochemical deposition ofAu and poly MnTAPP occurs together, respectively.

Here in Fig. 1C, at the initial stage, the electrodeposition of Auparticles occurs with the higher reduction currents. Also inFig. 1A at the initial scan range the reduction current appears high-er than the only poly MnTAPP electrodeposition (Fig. 1B). There-fore, the higher reduction current is due to the initiation of Auelectrodeposition process which exhibits at the initial scan, respec-tively [15]. As a result, in the hybrid film (poly MnTAPP and nanoAu) the redox peaks of poly MnTAPP not that much as visible aslike only poly MnTAPP. Here the redox couple with the formal po-tential of E00 = 0.25 V indicates the formation of polymer bridged

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ectrodeposited poly MnTAPP-nano Au (view angle: 90�), (c) poly MnTAPP-nano Au

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functional groups of MnTAPP [9,10]. The poly MnTAPP-nano Aufilm modified GCE was washed with deionized water and exam-ined for the electrochemical studies. Based on the previous litera-ture reports, pH 10.4 buffer solution was found as suitable for thedetection of NaOCl [7]. Therefore, we have selected this buffersolution for the detailed electroanalysis of NaOCl.

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Fig. 3. (a) Tapping mode AFM image of poly MnTAPP film, (b) poly MnTAPP-nanoAu film on ITO.

Table 1AFM analysis.

AFM parameters Bare ITOa Poly MnTAPPmodified ITOa

PolyMnTAPP-nanoAu modified ITOa

Average roughness(AR) (nm)

2.19 (s.d = 0.737) 3.82 (s.d = 1.58) 26.9 (s.d = 4.68)

Standard deviation (s.d).a (n = 5).

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3.2. SEM, AFM and UV–vis analysis

Fig. 2a SEM image shows the electrodeposited poly MnTAPP.From the SEM image (Fig. 2a), we can reveal that the poly MnTAPPhas been deposited as a thin layer on the electrode surface. Fig. 2band c shows the 90� and 60� view of the electrodeposited polyMnTAPP-nano Au modified GCE. This figure clearly depicts thepresence of poly MnTAPP as a thin layer and nano Au as particleson the electrode surface. Here the electrodeposited nano Au parti-cles formed as a group on the electrode surface. Further the groupof nano Au particles size varies in the range of 170–350 nm. Allthese SEM results show the surface nature of the electrodepositedpoly MnTAPP and nano Au particles.

Next the poly MnTAPP, poly MnTAPP-nano Au film modifiedITOs have been examined using tapping mode in AFM. Fig. 3aand b shows a typical AFM 2D image of the electrodeposited polyMnTAPP and poly MnTAPP-nano Au film. From Fig. 3a it can beseen that the electrodeposited poly MnTAPP film has been foundas distorted thin layer on the electrode surface. At the same time,poly MnTAPP film modified with nano Au displays the group ofnano Au particles very clearly on the ITO surface. This result vali-dates the deposition of both poly MnTAPP and nano Au on theITO. Further the AFM analysis similarly resembles and corroboratewith the SEM results. A detailed surface morphological study hasbeen done for the above mentioned films and various surfaceparameters have been analyzed using AFM.

Table 1 shows the several AFM image analysis (n = 5) results ofthe bare, poly MnTAPP and poly MnTAPP-nano Au modified ITO.The average roughness’s (AR) of these ITOs have been examinedseveral times and the average values have been given in theTable 1. From this Table 1 result we can see the increase in theaverage roughness (AR) value from bare to poly MnTAPP and polyMnTAPP-nano Au modified ITO. These results validate the presenceof both poly MnTAPP and poly MnTAPP-nano Au on the ITO sur-face. Here the poly MnTAPP-nano Au film possesses the higherroughness average value comparing with the only poly MnTAPPfilm. This is because the presence of group of nano Au particles.Also, the only poly MnTAPP film modified ITO’s average roughnessis higher than the bare ITO. This confirms the electrodeposition ofpoly MnTAPP on the ITO, respectively. Based on the SEM and AFManalysis, the maximum diameter of the group of nano Au particlesare found in the size range of 170–350 nm. Here the averagediameter of the group of nanoparticles was found as 235 nm. Theaverage height of the electrodeposited nano Au particles was foundas 70 nm.

In the next step, UV–vis spectroscopy has been employed forthe analysis of poly MnTAPP-nano Au modified ITO. Fig. 4 showsthe UV–vis absorption spectra of the poly MnTAPP-nano Au elec-trodeposited ITO (Inset of Fig. 4 shows the poly MnTAPP electrode-posited ITO). For the poly MnTAPP-nano Au modified ITO, UV–visabsorption peaks appear at 475, 520, 575 and 805 nm. At the sametime, for poly MnTAPP modified ITO the absorption peaks appearsat 435, 475, 565 and 637 nm (Inset of Fig. 4). Here the absorptionpeaks differ in the hybrid film because the presence of both polyMnTAPP and nano Au on the ITO surface. Finally, the SEM, AFMand UV–vis spectroscopy results clearly show the surface morpho-logical properties and the presence of the electrodeposited polyMnTAPP-nano Au film.

3.3. Amperometric detection of NaOCl

Electrocatalytic response of poly MnTAPP-nano Au film (NaOCldetection) has been compared with bare and poly MnTAPP modi-fied GCE (Fig. 5A). Fig. 5A curve (c) shows the CV response of polyMnTAPP-nano Au film modified GCE in pH 10.4 NaOH (absence ofNaOCl). For the 0.01 M concentration range, in pH 10.4 NaOH, polyMnTAPP-nano Au film modified GCE exhibits NaOCl electrochemi-cal signal at around �0.15 V (Fig. 5A, curve a). At the same time,the poly MnTAPP film modified GCE (Fig. 5A, curve b) and the bareGCE (Fig. 5A, curve a0) does not show any enhanced current re-sponse for the detection of NaOCl. Only, the poly MnTAPP-nanoAu film shows the enhanced reduction current response at�0.15 V. Based on this, it proves that poly MnTAPP-nano Au film

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Table 2Comparison table for sodium hypochlorite detection.

Type Buffer pH Potential(V)

Range Ref.

Green method NaOH 10.4 – 0.07–0.42g Cl2 l�1

[7]

FIA system NaCl (10 mM) 5.5 0.2 5 lg/ml [17]Oxidized GCE �0.2 –

NaCl (0.1 M) – [18]Dendrimer-Au

GCE– –

Pt indicatorelectrode

Acetatebuffer (0.1 M)

4.7 – 3.7–60 lM [19]

Poly MnTAPP-nano Au GCE

NaOH 10.4 �0.1 2.47 � 10�5–1.07 � 10�2 M

Thiswork

Fig. 4. UV–vis absorption spectra of poly MnTAPP-nano Au modified ITO (Insetshows the UV–vis absorption spectra of poly MnTAPP modified ITO).

Fig. 5. (A) CV response for NaOCl detection (0.01 M) at (a0) bare, (a) poly MnTAPP-nano Au, (b) poly MnTAPP modified GCE and (c) CV response of poly MnTAPP-nanoAu film modified GCE in pH 10.4 NaOH (absence of NaOCl). (B) Amperometric i-tresponse of the poly MnTAPP-nano Au film modified GCE in pH 10.4 NaOH for thesequential additions of NaOCl (2.47 � 10�5–1.07 � 10�2 M, potential: �0.1 V). Theinset shows the magnified view of the NaOCl detection.

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modified GCE is suitable for the electrocatalytic reduction of NaOClcomparing with the other electrodes (bare and poly MnTAPP mod-ified GCE.

Therefore, increase in the reduction peak current and decreasein the over potential at poly MnTAPP-nano Au show that the pro-posed film modified GCE successfully detects the NaOCl.

ClO� þH2Oþ 2e� ! Cl� þ 2OH� ð1Þ

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It is more complicated to predict the exact reaction mechanismfor the NaOCl reduction and the electrode reaction mechanism forthe proposed film. Also, here this film has been focused and devel-oped for the NaOCl electro reduction process. Therefore, based onthe previous literature reports [16,17] here we propose a generalelectrocatalytic reduction mechanism for the NaOCl reduction.This mechanism is purely based on the previously literature re-ports which says that the chlorine may present in two forms inwater. They are HOCl and ClO�. At the same time in higher pH con-ditions (>8.5) ClO� will be the dominant in water. Here we use pH10.4 NaOH as buffer solution. Therefore, the above provided mech-anism will be the expected general mechanism for the electrocat-alytic reduction of NaOCl, respectively.

To study the detailed electrocatalytic activity of NaOCl reduc-tion, amperometric i-t method has been employed for the laband real sample analysis. Fig. 5B shows the amperometric responseof poly MnTAPP-nano Au film modified GCE in pH 10.4 NaOH forthe different concentrations of NaOCl. Here for each 1.5 min theNaOCl has been added in the buffer solution (pH 10.4 NaOH,�0.1 V) and the current response has been noted.

For the every additions, the reduction current was increasingwith respect to the increasing concentrations of NaOCl and exam-ined up to the value of 1.07 � 10�2 M. The amperometric currentresponse for NaOCl reduction reached within 5 s following withthe additions of NaOCl and this current response were directlyproportional to the NaOCl concentrations, respectively. Based onthe i-t current response we partially assume that the initial con-centration of NaOCl (2.47 � 10�5 M) will be the detection limit ofthis film. From this result, the electrocatalytic activity of polyMnTAPP-nano Au film modified GCE for NaOCl detection has beenvalidated. Table 2 shows the comparison study of the NaOCl detec-tion with previous studies. Here the proposed film shows the suf-ficient range of detection of NaOCl comparing with other reports,respectively.

3.4. Amperometric detection of NaOCl in real samples and interferencestudies

In the next attempt, poly MnTAPP-nano Au film modified GCEdirectly employed for the detection of NaOCl in real samples(Fig. 6A). Commercially available bleaching agent (containing 5%Cl) has been examined. For every 1.5 min the NaOCl has beenadded in the NaOH buffer solution (pH 10.4 NaOH, at: �0.1 V)and the current response has been noted. The poly MnTAPP-nanoAu film successfully shows the amperometric response for thedetection of NaOCl in higher concentrations ranges. As expected,the poly MnTAPP-nano Au film modified GCE shows theamperometric current response for the NaOCl detection within5 s for the additions of bleaching solution in pH 10.4 NaOH.

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Fig. 6. (A) Amperometric i-t response of poly MnTAPP-nano Au film modified GCE in pH 10.4 NaOH for the sequential additions of NaOCl (2.47 � 10�5–1.07 � 10�2 M,potential: �0.1 V). The inset shows the magnified view of the NaOCl detection. (B) Amperometric current response of the poly MnTAPP-nano Au film for the each sequentialadditions (100 lL of each concentration 1.2 � 10�3 M) of NaOCl, H2O2, C2H5OH, Na2S2O4, H2O2 and C2H5OH in pH 10.4 NaOH.

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In the next step, the interference studies have been examinedwith five different species: NaOCl, H2O2, C2H5OH, Na2S2O4, H2O2

and C2H5OH (100 lL of each concentration 1.2 � 10�3 M)(Fig. 6B). Here the poly MnTAPP-nano Au film exhibits the currentincrease for the sequential additions of NaOCl. At the same time,for the additions of H2O2 and ethanol, poly MnTAPP-nano Au filmdoes not show the interference current response. However, forNa2S2O4 addition, it showed a small decrease in the current value.Further for the continuous additions of NaOCl, the current re-sponse were increasing and for the continuous additions ofH2O2 there was no interference current at the poly MnTAPP-nanoAu film modified GCE. This shows that poly MnTAPP-nano Au filmsuccessfully overcomes the interference signals and show only

the NaOCl detection signals, respectively. The repeatability andreproducibility nature of the modified GCE has been examinedfor the fixed concentration of NaOCl and in the five other GCEsand the relative standard deviation (% RSD) for these measure-ments were found as 4.43% and 3.52%. Further the stability ofthe film has been examined by keeping the modified GCE exposedin open air condition at the room temperature. After 1 day thebackground study have been done and there is 2% decrease fromits initial value. This shows that the proposed film holds the en-ough stability. Based on this stability examination, we assumethat the shelf-life of this film modified GCE will be 3 days whichis based on the decrease in the background current response inpH 1.0 H2SO4 solution.

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4. Conclusion

Here we propose a simple one step electrochemical depositionmethod for the fabrication of poly MnTAPP-nano Au film on GCEand ITO surface. The poly MnTAPP, poly MnTAPP-nano Au films havebeen characterized using SEM, AFM and UV analysis. Poly MnTAPP-nano Au film was found as electrochemically active and stable. Theproposed film modified GCE successfully employed for the detectionof NaOCl. Not only limited to lab sample, poly MnTAPP-nano Au filmmodified GCE successfully detects the NaOCl in commerciallyavailable real samples in the wide concentration ranges. Also, thisfilm successfully overcomes the interferences and shows only theelectrochemical signal of NaOCl, which indicates the special natureof this film. In conclusion, the poly MnTAPP-nano Au film could beemployed as prototype sensor for the detection of NaOCl.

Acknowledgment

This work was supported by the National Science Council of theTaiwan (ROC).

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